This invention relates to temperature-controlled electronic devices such as temperature-controlled resistance strain gauge bridges.
The concept of controlling the temperature of an electronic device to minimize a change in the characteristics of the device with temperature, hereinafter referred to as thermal shifts, is well known.
A known pressure transducer comprises a silicon diaphragm having a four-arm, active, resistance strain gauge bridge formed thereon, the four resistors of the bridge being diffused into the silicon diaghragm at the areas of maximum strain. A constant current, or a current fed by way of a resistor from a source of a constant voltage is applied to opposed first and second junctions of the bridge. If the resistance of the resistors in the bridge remains constant the voltage between the first and second junctions also remains constant. A strain in the diaphragm due to an applied pressure or force results in a change in the resistance of the bridge resistors and this change is detected by detection means coupled to the third and fourth opposed junctions. The detection means would often include an amplifier and the output of the bridge could be expressed in a current per unit pressure or force.
If the temperature of the device remains constant then the resistance of the resistors and hence the output of the bridge for a given strain in the transducer would remain constant provided that the strain coefficients of the bridge strain gauge resistors are matched.
However, if the temperature of the diaphragm changes due to changes in operating and/or ambient temperature then the values of the bridge resistors will change with a corresponding change in the voltage across the first and second junctions of the bridge and this is manifested as an error in the measured output signal.
Any change in temperature could also affect the operation of other associated devices, such as the measuring amplifier which would result in a change in the amplifier offset voltage, drift in the amplifier output signal and the like.
It is therefore important to maintain constant the temperature of such electronic devices or to provide some means of compensating for temperature change.
One known pressure transducer comprising a silicon diaghragm and a four arm active bridge as hereinbefore mentioned includes two other sets of silicon resistors deposited on the substrate in a non-active area. One temperature measuring set of the resistors is arranged to provide a signal representative of the temperature of the diaphragm to the input of an amplifier, the output of which is arranged to provide an output current to the other, heating set of resistors thereby to heat the diaphragm and to maintain it at a predetermined temperature. A control feedback loop is provided in known manner between the output and input of the amplifier to maintain the temperature substantially constant and to prevent fluctuations about the predetermined temperature.
This known arrangement maintains the temperature of the diaphragm approximately constant and reduces errors in the output of the bridge. However, as the temperature control is determined by the temperature sensing resistors on the non-active area of the diaphragm, temperature gradients across the silicon substrate and variations of these temperature gradients with ambient temperature limit the accuracy to which the temperature of the active area of the diaphragm can be controlled.